The present invention relates to a neuroprosthetic device for activating the body with functional electrical stimulation (FES), and more particularly, to a surface neuroprosthetic device that enables the device user facile on-line adjustment and fine-tuning of the local current density over the surface of the scanning electrode, so as to achieve optimal muscle response.
Neuroprostheses and therapeutic FES devices, based on surface stimulation, typically interface with the body limb through an array of surface electrodes positioned over the limb surface. Electrical stimulation delivered to the underlying limb musculature and neurological structures through the surface electrode array causes activation of the muscles, and controlled movement of the limb. Such devices are used for restoring active function to paralyzed or plegic body limbs in patients suffering disease or trauma to the central nervous system, in neurological conditions such as stroke, spinal cord injury, head injury, cerebral palsy and multiple sclerosis. Surface neuroprostheses use controlled electrical currents through electrodes placed on the surface of the body, in order to elicit contraction of selected muscles or to input sensory stimulus. Neuroprostheses can activate paralyzed muscles of the limb in an independent fashion, or in coordination with voluntary activation of muscles under natural neurological control. These devices are in use today for functional activities such as walking, standing, gripping or releasing objects, and are used both as a therapeutic modality and for improvement or restoration of activities of daily living.
The aspiration to facilitate the positioning of stimulating electrodes of FES devices over the activation points of impaired limb has evoked, in the prior art, the design and manufacture of devices that substantially conform to the shape of particular body sites and limb. Accurate positioning of the electrodes enables optimal muscle activation to give correct movement of the limb with minimum discomfort and fatigue. Typical examples of devices for stimulating particular body sites are Liberson et al., Arch. Phys. Med., 1961, 42: 101–105 and U.S. Pat. No. 4,697,808 to Larson, et al., for activating the lower limb, and U.S. Pat. No. 5,330,516 to Nathan and U.S. Pat. No. 5,562,707 to Prochazka, for activating the wrist or forearm.
The contact area of the surface electrode is an important factor in the performance of a neuroprosthesis device. Large surface area electrodes tend to disperse the stimulation field over a large skin area, and the stimulation current density passing through the skin is relatively low, resulting in relative sensory comfort. In this case, however, the resolution of the electrode is correspondingly low, as a relatively large region of excitable tissue immediately underlying the electrode may be activated (see Sagi et al., 3-D Current Density Distributions Under Surface Stimulation Electrodes, Med. & Biol. Eng. & Comp., 33, pp. 403–408, 1995).
Earlier electrodes, such as set forth in U.S. Pat. No. 4,736,752 to Munck, et al., teach the control of the current density across the electrode through the use of conductive ink and adhesive patterns. The deficiencies of such electrodes are manifest from the description hereinbelow.
It should be emphasized that accurate electrode placement is very important for surface neuroprostheses. The patient is required to ensure, each time he wishes to set up the neuroprosthesis device, that all the electrodes are positioned accurately over the motor points of the muscles to be activated. Even slight deviations in the placement of the electrode may deleteriously effect the response of the limb. Alternatively or additionally, such deviations from the proper positioning may cause undesired movements, discomfort and unnecessary fatigue. This phenomenon is particularly pronounced at certain body limb sites that have a number of different excitable tissues disposed within a small region, within which a controlled current must be applied. These systems often have electrode placement problems, because the stimulating electrode is relatively large and consequently, does not enable precise activation by selectively focussing on a small target region of excitable tissue while avoiding excitation of unwanted tissue underlying the electrode
This phenomenon, which may be termed “crowding” of excitable tissue, is particularly problematic at two limb sites: the dorsal surface of the forearm, and the dorsi-flexor surface of the lower leg. In these locations, small variations in electrode placement tend to generate large changes in hand and foot posture respectively.
U.S. Pat. No. 6,038,485 to Axelgaard discloses a transcutaneous medical electrode. The electrode has a highly conductive grid including a plurality of arrays of electrical conductors (conductive inkspots) for controlling current distribution of directed electrical pulses. Electrical connectors are provided for establishing electrical communication with the conductive grid for switching ON or OFF the electrical conductors in each array. The conductive grid is supported by a moderately conductive sheet, or film, and a conductive adhesive is provided for removably coupling the sheet or film and the conductive grid to the body of a user. The device is configured such that the conductive ink spots can be switched on and off so as to control the local current density across the electrode.
In addition, U.S. Pat. No. 6,038,485 teaches thickening of the support layer in areas that a reduced current density is needed, or thinning of the support layer in areas that require a higher current density. However, this adjustment procedure requires removal of the electrode and considerable technical knowledge and experience. Adjustments of this electrode would clearly be carried out “off-line”, as the electrode is unsuitable for on-line adjustment of a neuroprosthesis. Moreover, the tuning of a device having such an electrode would be generally beyond the skill of the user, such that the ministrations of an expert in the field of FES would be required.
It would be highly desirable in neuroprostheses for the device to be adjustable, while the system is in use, so as to enable the use of direct feedback from the resulting limb posture and movements to guide the system user to adjust the system proprioceptively to achieve an optimal response.
In summary, there is no known neuroprosthetic device that enables the patient to tune the local current density delivered to the skin surface, while working with the device, and without the help of a clinician. There is, therefore, a recognized need for, and it would be highly advantageous to have, a neuroprosthetic device that enables the patient to tune the local current density delivered to the skin surface, with facility, and without the help of a clinician, so as to activate the muscles in an optimal manner.